The present disclosure relates to a back side illumination image sensor.
Image sensors are semiconductor devices that convert optical images into electric signals and are generally classified as a CCD (Charge Coupled Device) image sensor or a CMOS image sensor (CIS).
A photodiode of a related art CIS is formed on a substrate by an ion implantation process. As the size of the photodiode gradually decreases to increase the number of pixels without increasing the chip size, there is growing tendency that image quality is reduced due to reduction of the area of the illumination part.
Further, since the stack height is not reduced as much as the reduction of the area of the illumination part, there is tendency that the number of photons entering the illumination part is reduced by diffraction of light, which is called “airy disk”.
In order to address the above problem, there is provided a back side illumination image sensor that receives light through a wafer back side, minimizing the step at the upper portion of the illumination part and removing interference of light due to metal routing.
FIG. 1 is a cross-sectional view illustrating a process of a back side illumination image sensor according to the related art.
In the back side illumination image sensor according to the related art, an illumination device and a wiring are formed on the front side of a substrate, and then a back grinding for removing the rear side of the substrate to a predetermined thickness is performed. This back grinding process of the rear side of the substrate is for fitting the gap between an external module and an optical lens to an appropriate thickness.
However, in the back side illumination sensor according to the related art, an SOI (Silicon On Insulator) wafer is used as a donor wafer where an illumination device and a circuit part are formed, and then the SOI wafer is bonded to a handle wafer. Thereafter, a back side thinning process is applied to the donor wafer.
The backside thinning process that is applied to the donor wafer according to the relate art is as follows.
First, the backside grinding process is applied to the donor wafer such that several tens of μm remains on the upper portion of a BOX (Buried Oxide) layer of the SOI wafer. Thereafter, the backside thinning process is completed by performing an etch-back process.
However, according to the related art, since an expensive SOI wafer is used for the donor wafer, the cost for manufacturing process increases.
Further, according to the related art, as shown in FIG. 1, wafer edge thinning is generated by the backside grinding process of the donor wafer. Accordingly, a fail may occur in the chip at the wafer edge in the etch-back process performed after the backside grinding process, resulting in a problem that economical efficiency is considerably deteriorated.
Further, according to the related art, the wafer center is also exposed to plasma damage in the etch-back process of several tens of μm, resulting in a problem that sensor performance may be deteriorated.
Further, according to the related art, it is required to remove the portion of the substrate covering and corresponding to a photodiode on the back side of a silicon substrate.
However, a large number of defects are generated on the surface of the substrate by removing the back side of the silicon substrate. Therefore, characteristics relating to leakage are deteriorated, which deteriorates CIS image quality.
Meanwhile, according to another related art, the photodiode may be deposited by using amorphous silicon. Otherwise, a readout circuit is formed on a silicon substrate, the photodiode is formed on another wafer, and then the photodiode is formed over the readout circuit by wafer-to-wafer bonding to form an image sensor (hereinafter referred to as “3D image sensor”). The photodiode and the readout circuit are connected by a metal line.
However, according to the 3D-image sensor of the related art, when bonding the wafer having the readout circuit to the wafer having the photodiode, it can be difficult to completely electrically connect the readout circuit with the photodiode due to a problem relating to bonding. For example, according to the related art, a metal line is formed on the readout circuit and wafer-to-wafer bonding is performed such that the metal line contacts with the photodiode, but the metal line may not completely contact with the photodiode. Therefore, it can be difficult to implement an ohmic contact between the metal line and the photodiode. Further, according to the related art, a short may be generated in the metal line that is electrically connected with the photodiode. Accordingly, researches for preventing the short have been conducted, but the process becomes complicated.